If I understand correctly, you have the coordinates of 2 points that define a line (let's call them x1,y1 and x2,y2) and you want to know the coordinates of a point x,y that is a certain distance d along the line from x1,y1 in the direction of x2,y2. If that's what you are looking for, following methodology applies.

1. The line is at an angle to horizontal. The tangent of the angle (theta) is: tan(theta) = (y2 - y1)/(x2 - x1). Calculate theta

2. The unknown point x,y is a distance d along the line from point x1, y1 in the direction of x2,y2.
x = x1 + d*cos(theta)
y = y1 + d*sin(theta)